The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium which is markedly improved in adhesion between the substrate polyester film and the electron radiation cured magnetic layer by providing therebetween a coating layer comprising a polyurethane having anionic group or a mixture of a polyurethane having anionic group and a polyester having sulfonic group.
Biaxially oriented polyester films, especially biaxially oriented polyethylene terephthalate films have been popularly used as substrates for magnetic tapes, magnetic cards, magnetic discs, etc., due to their excellent mechanical strength, dimensional stability, heat resistance, chemical resistance and other advantageous properties. In the magnetic layer of conventional magnetic recording media, a heat-reactive compound such as a polyisocyanate has been blended as a binder for improving the adhesion of the magnetic layer to the substrate. The conventional heat-reactive compounds, however, have required not only heating but also long-time aging for effecting satisfactory curing. Also, some types of heat-reactive compounds such as polyisocyanates are cured even at normal temperature in the presence of water, so that great care must be given to the adjustment of water in the magnetic coating material or the prevention of its deterioration in the course of time in storage. As a measure to eliminate such difficulties in treatment of the conventional heat-reactive compounds as possible resultant scattering of properties of the produced magnetic recording media and the rise of cost due to the necessity of using much time and energy, various proposals and studies have been made recently on the use of the compounds curable with active rays such as ultraviolety rays or electron radiations as a binder for the magnetic layer. As the heat-reactive compounds curable with such active rays, there have been used the compounds having vinyl group as reactive group, but such compounds, when polymerized into a polymeric compound, would prove unsatisfactory in adhesiveness to the substrate polyester film. For improving the adhesion between the polyester film and the magnetic layer, proposals have been made on introduction of a new component to be bonded with the reactive group in the heat-reactive compound or combined use of other type of binder. However, the magnetic recording media using the hitherto proposed binders were still unsatisfactory in adhesion between the substrate polyester film and the magnetic layer and therefore not freed of such problems as improper dispersion or orientation of the magnetic material and deterioration of mechanical strength and magnetic properties of the magnetic layer. Thus, a magnetic recording medium having good adhesion between the substrate polyester film and the magnetic layer has been desired.
The present inventors have pursued studies for providing a magnetic recording medium markedly improved in adhesion between the substrate polyester film and the magnetic layer and as a result, it has been found that a magnetic recording medium obtained by forming on both sides or one side of the substrate polyester film a coating layer comprising a polyurethane having anionic group or a mixture of a polyurethane having anionic group and a polyester having sulfonic group, and then forming a magnetic layer, especially an electron radiation cured magnetic layer on the coating layer, was remarkedly improved in adhesion between the substrate polyester film and the magnetic layer as compared with the conventional magnetic recording media. The present invention has been attained on the basis of this finding.